1. Field of the Invention
The present invention relates to an image recording material usable as an offset printing master and a novel ammonium compound used for the recording material, and, particularly, to a positive image forming material useful as a positive planographic printing plate precursor for an infrared laser which planographic printing plate precursor is used for direct plate-making from digital signals from, especially, computers and also to a novel ammonium compound useful for improving the alkali resistance of the image forming material.
2. Description of the Related Art
The recent development of lasers are conspicuous and as, particularly, solid lasers/semiconductor lasers having an emission range from the near infrared region to the infrared region, high-output and small-sized lasers are becoming easily available. These lasers are very useful as exposure light sources when plate-making directly from digital data of computers and the like.
In positive light-sensitive image informing materials for infrared lasers which materials are used for direct plate-making and have long been well-known, novolac resins are used as an aqueous alkali solution-soluble resin. For instance, the positive light-sensitive image forming material disclosed in the publication of Japanese Patent Application Laid-Open (JP-A) No. 7-285275 is a type in which a material which absorbs light to generate heat and a positive light-sensitive compound such as various onium salts and quinonediazide compounds are added to an aqueous alkali solution-soluble resin, such as a novolac resin, having a phenolic hydroxyl group, wherein the positive light-sensitive compound works as a dissolution inhibitor which substantially lowers the solubility of the aqueous alkali solution-soluble resin in image portions and does not eventually manifest the dissolution inhibitive function due to heat in non-image portions, so that the non-image portions are made removable by developing, thereby forming an image.
Also, the positive light-sensitive image forming materials described in WO97/39894 and EP0823327A2 comprise a material which absorbs light to generate heat and a rein which is changed in solubility in an aqueous alkali solution by heat, wherein these image forming materials have low solubility in an aqueous alkali solution in image portions and have increased solubility in an aqueous alkali solution by heat in non-image portions, so that the non-image portions are eventually removable by developing, whereby an image is formed.
In current planographic printing plates, a novolac resin is used particularly preferably from the reasons that there is a large difference in solubility to the developing solution between exposed portions and unexposed portions because it strongly interacts with a dissolution inhibitor and that the resin has high ink-accepting capability. And, with regard to the positive light-sensitive image forming materials for infrared lasers, a novolac resin is also used from the same reasons.
However, a heat mode positive image forming material in exposure using an infrared laser poses the problem that a difference in solubility to the developing solution (dissolution discrimination) between exposed portions and unexposed portions is insufficient and excess developing and developing inferiors are easily caused by variations in working conditions. Also in the case where the surface condition is slightly changed by touching the surface during handling, unexposed portions (image portions) dissolve in a developing solution to leave a scar-wise portion, causing deterioration in printing durability and print-adhesion inferior, leaving fears for affecting various performances such as processing stability, development latitudes and scratching resistance.